The present invention relates to an arrangement and a method relating to speech transmission wherein the transmitted signals are divided into a frame structure. The invention also relates to a telecommunications system comprising an arrangement relating to speech transmission.
In digital telecommunications systems a frame structure is almost always used and speech is transmitted in speech (traffic) frames. A frame here relates to an information block comprising a given number of digital information bits. When speech is to be transmitted the solution is not straightforward since on one hand both speech and background noise, which may vary to a great extent, is present and on the other hand a human speaker normally does not speak uninterruptedly but now and then makes pauses and remains silent. Furthermore, frames or speech-frames may be bad, i.e. lost or corrupted during transmisson.
When a transmitted frame is bad or lost it will generally be replaced since normal decoding of such frames would produce noise effects which are very annoying for a listener.
GSM Recommendations GSM 06.11, October 1992, "Substitution and Muting of Lost Frames for Full-Rate Speech Channels" relates to muting when the full-rate speech coding is applied, i.e. they define a frame substitution and muting procedure to be used by the receiving side when one or more lost speech frames or SID (Silence Descriptor) frames are received.
When speech frames have been lost, the speech volume is decreased. A muting technique is disclosed through which the output level is decreased gradually resulting in silencing of the output after a maximum 320 ms. This means that silence will be received after max 320 ms which can be very annoying since it is an abrupt change from speech plus background noise to silence. Often a period which is shorter than 320 ms is used in practice which can be even more annoying.
If aural information comprises both speech and background noise mixed, muting towards silence induces inconvenient sparkling. Thus, for a number of known muting algorithms which are applied on disturbed speech coding parameters, the background noise chops down to silence and this may happen more than once a second. Furthermore, known solutions do not take into account such situations when background noise is present such as babble, car-noises etc., which however are realistic traffic cases.